1. Field of the Invention
This invention relates to systems and methods for improving a power and bandwidth constrained transmission system, and in particular to a system and method for adaptively varying the transmission signal according to measured or predicted time-varying signal propagation characteristics.
2. Description of the Related Art
A typical communications system, whether terrestrial or satellite-based, is constrained in its transmission power and signal bandwidth. The communications system may also be required to deliver reliable communications to one point, as in a terrestrial microwave link, or to many points in a satellite broadcast system serving the United States.
A typical satellite communications system servicing a large geographical region such as the United States is typically designed such that all signals transmitted to terrestrially based receivers have a minimum carrier to noise ratio (CNR) when received anywhere in the service area or coverage region. The CNR of the signal at the receiver is a function of a number of parameters, including the power of the signal transmitted from the satellite and signal propagation characteristics such as atmospheric attenuation. Unfortunately, at the wavelengths typically used in satellite-to-home television broadcasts, rain and other sources of atmospheric attenuation have a significant effect upon the CNR. Hence, during heavy rain, some customers can experience degraded signal quality.
In the past, system designers have assured a minimum signal quality is provided by designing the satellite transmitters, power systems, and related components to produce sufficient power to assure a minimum CNR in all regions, even when compromised by unfavorable atmospheric attenuation characteristics. Unfortunately, the amount of attenuation from rain and similar atmospheric parameters can be substantial and widely varying. Hence, to achieve the desired CNR in all areas (including those with intense rain) this solution requires that the satellite transmitters, power system and related components be designed for high performance levels.
Increasing transmitter power on a communications satellite may be economically impractical or even technologically infeasible. Satellites have severe limitations on weight which can be delivered at desired orbital locations due to launch vehicle costs and other limitations. This weight limitation constrains the size of solar panels, the charge capabilities of the on-board batteries, and the physical size of the transmitter and related electronics. Furthermore, one type of transmitter, the traveling wave tube amplifier (TWT) has upper power limits for reliable operation.
Systems have been defined for controlling transmitter power to account for variable weather conditions, including U.S. Pat. No. 6,587,687 for a “Multiple Satellite Fade Attenuation Control System,” U.S. Pat. No. 6,421,528 for a “Satellite Transmission System with Adaptive Transmission Loss Compensation,” and U.S. Pat. No. 5,878,328, for “Power Control of an Integrated Cellular Communication System.” However, the solutions offered by these systems do not teach selecting signal transmission modes defined by characteristics such as modulation type, coding, and coding rate to adaptively control the transmission system either alone or as an adjunct to varying the transmitter power. Hence, allocation of system transmission resources in such systems are typically sub-optimal.
What is needed is a system and method that ameliorates the foregoing limitations. The present invention satisfies that need.